Fabrication of an invertedly poled domain structure from a ferroelectric crystal

ABSTRACT

A method of fabricating an invertedly poled domain structure having alternating sections of opposite electric polarities, from a ferroelectric crystal wafer ( 1 ) having two opposite polar surfaces, comprises patterning at least one of the two polar surfaces of the wafer to comprise a plurality of alternating discrete regions, of which first regions are adapted for and second regions are protected from the direct application thereto of an electric contact; applying to both polar surfaces of the wafer electrically conducting electrodes ( 10  and  11 ) so that the first regions are in direct contact with the electrodes and the second regions are protected from such a contact; and applying to the electrodes an electrical field ( 20 ) of the intensity E. The electrical field is applied to the wafer at a working temperature by heater/cooler ( 15 ).

CROSS REFERENCE TO RELATED APPLICATION

The present application is the national stage under 35 U.S.C. 371 ofPCT/IL98/00054, filed Feb. 4, 1998.

FIELD OF THE INVENTION

The present invention refers to a method of fabricating controlleddomain structures in ferroelectric materials where domains of differentsections of the structure have different polarities. Ferroelectricstructures of this kind are used in applications where it is required tochange properties of electromagnetic radiation, for example, innon-linear optical converters where a fundamental radiation having onefrequency is converted into a radiation having another frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show how thesame may be carried out in practice, reference will now be made by wayof example only, to the accompanying drawings, in which

FIG. 1 is a schematic illustration of an invertedly poled domainstructure fabricated by a method of the kind to which the presentinvention refers;

FIG. 2 is a schematic illustration of a wafer in which one polar surfaceis formed with a patterned layer of an isolating material;

FIG. 3 is a schematic illustration of a wafer in which, adjacent onepolar surface of the wafer, a structure of chemically modified regionsis created;

FIG. 4 is a schematic illustration of a wafer in which, adjacent onepolar surface of the wafer, a structure of shallow inverted domains iscreated;

FIG. 5 is a schematic illustration of a vacuum chamber in which themethod according to the preferred embodiment of the present invention isperformed;

FIG. 6 illustrates the dependence on the temperature, of the dielectricresponse time and the switching time of a flux grown KTP, at theelectric field of 65 kV/cm;

FIG. 7 is an optical microphotographic representation of a cross-sectionof a periodically invertedly poled domain structure made of a flux grownKTP crystal by means of the method of the present invention; and

FIG. 8 illustrates the dependence of the intensity of the generatedsecond harmonic intensity obtained with a periodically poled domainstructure fabricated by means of the method of the present invention, onthe wavelength of the fundamental radiation.

BACKGROUND OF THE INVENTION

Optical converters of the kind specified are used as sources of coherentradiation for such applications where, for example, laser sources of arequired radiation frequency are unavailable or where the turnability ofa source of radiation is required in a relatively wide range offrequencies. To achieve efficient optical frequency conversion, phasepropagation velocities of fundamental and converted radiation must beequalized, i.e. phase matching should be provided.

The optical conversion of the kind specified is based on nonlinearelectronic polarization which may be observed in crystals having nocenter of symmetry. Some crystals of this kind, namely ferroelectriccrystals, are characterized by having an electrical spontaneouspolarization P_(s). The sign of their nonlinear optical coefficient andtheir electro-optic coefficient depends on the direction of the vectorof this spontaneous polarization. The direction of the spontaneouspolarization can be reversed by applying to the crystal an externalelectric field stronger than the crystal's threshold defined as acoercive field. This possibility to selectively switch the spontaneouspolarization P_(s) in the crystals makes the ferroelectric materialsspecifically suitable for the fabrication of structures havingalternating sections in which domains have opposite electric polarities.In the alternating sections of such structures, the nonlinear opticalcoefficient has opposite signs, by means of which a desired phasematching is obtained.

One of known phase-matching methods is quasi-phase matching which isassociated with such ferroelectric structures where the sections ofinvertedly poled domains are arranged periodically. These structures areknown as periodically poled domain structures (PPDS) and FIG. 1 hereinillustrates an example of such a structure.

The fabrication of ferroelectric structures of the kind specified may beperformed by different ways of which one is based on the application toa ferroelectric crystal wafer of an external electric field which isstronger than the coercive field of the crystal, which causes theinversion of a polar axis thereof. This method generally comprises thefollowing sequence of operations:

(a) patterning at least one of two polar surfaces of the wafer tocomprise a plurality of alternating discrete first and second regions,of which said first regions are adapted for and said second regions areprotected from the direct application thereto of an electric contact;

(b) applying to both polar surfaces of the wafer electrically conductingelectrodes so that the first regions of said at least one polar surfaceare in direct contact with the electrodes and the second regions of saidsurface are protected from such a contact; and

(c) applying to the electrodes an electric voltage to provide anelectrical field E equal to or stronger than the material's coercivefield E_(c) and weaker than the breakdown field E_(br) of the material.

Step (a) of the above method may be performed in different ways. Thus,there may be formed on said at least one polar surface of the wafer, apatterned layer of an isolating material, for example, such as shown inFIG. 2 herein or as disclosed in U.S. Pat. No. 5,526,173; or selectedregions of the wafer adjacent to the polar surface may be chemicallymodified so as to inhibit subsequent nucleation and growth of selecteddomains, such as disclosed in EP 689 941 and illustrated in FIG. 3herein.

The electrodes applied to the polar surfaces of the wafer in step (b) ofthe above method, in particular the electrode applied to the patternedpolar surface of the wafer, may be either in the form of a continuouslayer or in the form of an array of separate electrodes each directlyconnected with a corresponding first region.

The electric field E applied to the wafer in step (c) above will bereferred to herein as a “switching field” and the voltage by means ofwhich such a field is provided will be referred to as “switchingvoltage”. The purpose of step (c) is to cause the switching of thecrystal polarity in those sections of the wafer which are associatedwith said first regions, whilst in the sections associated with thesecond regions, the original polarity will be kept unchanged. For mostapplications, it is desired that the switching is performed in such amanner that interfaces between the sections having opposite polaritiesare parallel to each other and extend through the entire crystal bodyfrom one of its polar surfaces to the other.

Most of processes of fabrication of invertedly poled structures arepresently performed and the set up used therein is adapted for operationat room temperature. However, most of attempts to use these processesfor the fabrication of periodic ferroelectric structures, especiallywith fine pitches between the sections, from commercially availableferroelectric crystals, encountered serious uniformity problems whichconstitute the main obstacle for the commercial application of suchstructures especially for bulk optical devices enabling operations withhigh power optical fluxes.

The materials usually used for the production of structures ofinvertedly poled domains are highly isolating ferroelectric crystalssuch as LiTaO₃, LiNbO₃, KTiOPO₄ (KTP) and RbTiOAsO₄.

LiNbO₃ and LiTaO₃ are the most popular. However, these materials areknown to have, at room temperature, very high coercive fields and,therefore, require the application of extremely high switching fields,i.e. about 260 kV/cm. Such high fields lead to lateral displacement ofdomain walls which renders the fabrication of uniform structures havingsmall periods very difficult, if not impossible. Moreover, since theintensity of the electrical field depends on the applied switchingvoltage and on the wafer's thickness, the high switching electric fieldsmay be obtained either by the increase of the applied switching voltageor by the use of thin wafers. However, a max value of the applicableswitching voltage is limited by practical considerations which putextremely severe limitations on the maximal thickness of wafers. Thus,the maximal thickness which a crystal of LiNbO₃ or LiTaO₃, may have toobtain the complete polarity inversion at room temperature is about 0.5mm, whilst with greater thickness, the polarity inversion cannot beachieved.

In KTP crystals and in some of its isomorphs, the coercive fields atroom temperature are relatively low (about 25 kV/cm) and, therefore, forthe polarity inversion, they do not require the application of highelectrical fields. These crystals also have a higher optical damagethreshold than that of LiNbO₃ and are, therefore, more suitable foroptical conversion. However, in view of the fact that the electricalconductivity of most of the commercially available KTP crystals is toohigh (σ≈10⁻⁷+10⁻⁸ Ω⁻¹ cm⁻¹) for the electrical poling process, it isvery difficult to obtain the desired domain poling in the firstunprotected regions while ensuring the efficient protection in thesecond regions. Therefore, most attempts to use the relatively highlyconductive ferroelectric crystals such as flux grown KTP crystals, forthe fabrication of invertedly poled domain structures, resulted inextremely nonuniform structures.

Thus, it is the object of the present invention to provide a method offabrication of an invertedly poled domain structure of a ferroelectriccrystal wafer, where the disadvantages discussed above are overcome. Inparticular, it is the object of the present invention to provide acontrollable method of fabrication of an invertedly poled domainstructure having high quality, high uniformity and high resolution.

SUMMARY OF THE INVENTION

In the present specification and claims, the term “dielectric responsetime τ_(res)” is defined as τ_(res)=κε/σ where κ is the dielectricconstant, ε is the vacuum permittivity and σ is the electricalconductivity of a ferroelectric crystal. The term “switching timeτ_(sw)” means the time during which the complete domain polarityinversion occurs in the ferroelectric crystal at a predeterminedintensity of an applied electric field. Both the dielectric responsetime and the switching time depend on the temperature at which theelectric field is applied to the crystal and there exists a temperatureT_(x) at which the dielectric response time τ_(res) equals the switchingtime τ_(sw), above which τ_(res)<τ_(sw) and below which τ_(res)>τ_(sw).

According to a first aspect of the present invention there is provided amethod of fabricating an invertedly poled domain structure havingalternating sections of opposite electric polarities, from aferroelectric crystal wafer having two opposite polar surfaces, themethod comprising:

(a) patterning at least one of said two polar surfaces of the wafer tocomprise a plurality of alternating discrete regions, of which firstregions are adapted for and second regions are protected from the directapplication thereto of an electric contact;

(b) applying to both polar surfaces of the wafer electrically conductingelectrodes so that the first regions are in direct contact with theelectrodes and the second regions are protected from such a contact;

(c) applying to the electrodes an electrical field of the intensity Ewhich satisfies the condition E_(br)>E≧E_(c), E_(c) being the coercivefield of the ferroelectric crystal and E_(br) being the breakdown fieldof the ferroelectric crystal; characterised in that

said electrical field is applied to the wafer at a working temperatureT_(w) which satisfies the condition T_(min)<T_(w)<T_(x), where thetemperature T_(min) is the minimal temperature at which the intensity Eof the switching electric field still satisfies the conditionE_(br)>E≧E_(c).

Thus, by performing the step (c) above at the temperature T_(w) which islower than T_(x), it is ensured that the response time τ_(res) of theferroelectric crystal wafer is longer than its switching time τ_(sw).Thus, the method of the present invention is based on the control oftemperature at which the switching field is applied to the wafer, whichallows to fabricate high quality invertedly poled domain structures fromvarious ferroelectric crystals.

According to another aspect of the present invention, there is provideda method of fabricating an invertedly poled domain structure havingalternating sections of opposite electric polarities, from aferroelectric crystal wafer having two opposite polar surfaces, themethod comprising:

(a) patterning at least one of said two polar surfaces of the wafer tocomprise a plurality of alternating discrete regions, of which firstregions are adapted for and second regions are protected from the directapplication thereto of an electric contact;

(b) applying to both polar surfaces of the wafer electrically conductingelectrodes so that the first regions are in direct contact with theelectrodes and the second regions are protected from such a contact;

(c) applying to the electrodes an electrical field of the intensity Ewhich satisfies the condition E_(br)>E≧E_(c), E_(c) being the coercivefield of the ferroelectric crystal and E_(br) being the breakdown fieldof the ferroelectric crystal;

characterised in that

the temperature at which the electrical field is applied to the wafer isbelow room temperature.

In the context of the present invention, the term “switching timeτ_(sw)” means the time during which the complete domain polarityinversion occurs in the sections of the wafer associated with the first,unprotected regions of the polar surface thereof. The switching time infact defines the time which is required for the charge injected into thewafer by an external electrical current to reach the value of 2 Ps·S₁,where Ps is the electric spontaneous polarization of the ferroelectricmaterial and S₁ is a total surface area of the first regions. On theother hand, the dielectric response time τ_(res) defines the time duringwhich the charge drifted inside the crystal body between the polarsurfaces of the wafer at the second, protected regions reaches the valueof 2 Ps·S₂, where S₂ is a total surface area of the second regions.

For the complete domain polarity inversion in the wafer sectionsassociated with the first regions of the wafer, a time interval τ_(dur)during which the switching field is applied to the ferroelectric crystalwafer, must not be shorter than the switching time τ_(sw). On the otherhand, to avoid the switching in those sections of the wafer associatedwith the second, protected regions, the time interval τ_(dur) should besubstantially shorter than the dielectric response time τ_(res) of theprotected regions. Therefore, the time τ_(dur) should be chosen, inaccordance with the present invention, so as to satisfy the abovecondition τ_(sw)<τ_(dur)<τ_(res).

By one mode of the present invention, said working temperature T_(w) atwhich the electric field is applied to the electrodes is lower than roomtemperature. This mode is based on the realization that, in someferroelectric materials, in particular those having relatively highelectrical conductivity, the temperature T_(x) below which a dielectricresponse time τ_(res) of the crystal is longer than a switching timeτ_(sw) of the crystal at the chosen intensity E of the electric field,is lower than room temperature, whilst at room temperature, theirdielectric response time τ_(res) is shorter than the switching timeτ_(sw). Thus, for example, at the applied electric field in the rangefrom 25 kV/cm to 65 kV/cm, the dielectric response time τ_(res) of KTPcrystals at room temperature is about 30-300 μs whilst the switchingtime τ_(sw) is in the range of 1.66-0.075 ms. Consequently, at roomtemperature, it is practically impossible, with most commerciallyavailable KTP crystals, to choose the time interval τ_(dur) of theapplication of the electric field for which the condition stipulated inaccordance with the present invention is satisfied, which does not allowfor a controllable method of their fabrication providing high qualityferroelectric structures produced thereby.

By another mode of the present invention, the working temperature T_(w)is higher than room temperature and substantially lower than Curietemperature of the ferroelectric crystal. The second mode of the presentinvention is based on the fact that in ferroelectric materials having atroom temperature relatively low electrical conductivity, the temperatureT_(x) is higher than room temperature and the conditionτ_(sw)<τ_(dur)<τ_(res) may be satisfied even at temperatures which aremuch higher than room temperature. By virtue of the increase of thecrystal temperature, the coercive field thereof becomes weaker, wherebythe switching electric field applied to the crystal may be much lowerthan that required at room temperature. This allows for the productionof the structures with finer pitches between the sections havingdifferent polarities and at lower switching electric fields, and enablesthe use of thicker crystal wafers.

In accordance with still another mode of the present invention, which isspecifically dedicated to ferroelectric crystals, a dielectric responsetime τ_(res) of which at room temperature is shorter than a switchingtime τ_(sw), the method comprises a starting step of modifying thecrystal to change its conductivity and thereby to increase thedielectric response time τ_(res) so as to meet the conditionτ_(res)>τ_(sw).

Preferably, the time interval τ_(dur) meets the condition:

(1/3)τ_(sw)≦τ_(dur)≦(0.1/0.3)τ_(res)

The switching field applied to the wafer in all modes of the method ofthe present invention may be in the form of a sequence of pulses thetotal duration of which is defined by τ_(dur).

For the patterning of said at least one polar surface of the waferdifferent methods may be used such as forming, on one of the polarsurfaces, a patterned layer of an isolating material, or forming,adjacent to one of the polar surfaces, either a structure of chemicallymodified regions or a structure of shallow inverted domains.

Preferably, the operation (c) of the method of the present invention isperformed with the wafer being mounted on a temperature controlled stagein a vacuum chamber.

Preferably, the ferroelectric material used in the method of the presentinvention is either K_(1-x)Rb_(x)TiOP_(1-y)As_(y)O₄ (1≧x≦0, 1≧y≧0); orA_(1-x)B_(x)Ti_(1-z)Nb_(z)OP_(1-y)As_(y)O₄ (1≧x0, 1≧y≧0, 1≧z≧0), where Aand B are one of the alkaline elements; Na, K, Cs, Rb or H; orLiNb_(1-x)Ta_(x)O₃ (1≧x≧0); or KNb_(1-x)Ta_(x)O₃ (1≧x≧0).

The invertedly poled domain structures fabricated by the method of thepresent invention may be periodic and non-periodic and may be used fordifferent purposes associated with conversion of electro-magneticradiation. Periodically, poled domain structures fabricated by themethod of the present invention are particularly useful to providequasi-phase matching for their use, for example, in second harmonicgenerators, sum frequency generators, difference frequency generators,optical parametric oscillators and the like.

DETAILED DESCRIPTION OF THE PREFERRED MODES OF THE METHOD OF THEINVENTION

FIG. 1 is a schematic illustration of an invertedly poled domainstructure 1 produced by a method of the kind to which the presentinvention refers. As seen, the structure 1 is periodic and has a regulardomain configuration in which a vector of spontaneous polarization P_(s)has opposite directions in adjacent sections 2 and 3 of the structure.The structure has a period of modulation Λ and is adapted to convert aradiation having a fundamental frequency ω into a radiation having asecond harmonic frequency 2ω.

To fabricate the structure as shown in FIG. 1 from a ferroelectriccrystal wafer having two polar surfaces 6 and 7, at least one polarsurface of the wafer, namely the polar surface 6, is first patterned tohave alternating first and second regions 8 and 9 such that the firstregions 8 are adapted for and the second regions 9 are protected fromthe direct application thereto of an electric contact.

The patterning of the polar surface 6 may be performed by one of thefollowing ways described with reference to FIGS. 2, 3 and 4.

FIG. 2 illustrates the polar surface 6 of the wafer, coated with a thinisolating layer (0.5-1.0 μm) of a photoresist material 5 patterned bymeans of any known microlithographic techniques. Thus, the photoresist 5covers only the regions 9 of the wafer, thereby protecting these regionsfrom the direct application thereto of an electric field. Alternatively,the photo-resist may be replaced by such isolators as, for example, SiO₂or Si₃N₄.

FIG. 3 illustrates the wafer modified chemically in the regions 9 so asto inhibit domain nucleation and growth. This method is disclosed in EP687 941 incorporated herein by reference.

FIG. 4 illustrates the wafer having a shallow pattern of inverteddomains in the regions 9. This method is generally based on thefabrication, in a ferroelectric material, of a bi-domain structureconsisting of two adjacent domains of opposite polarity oriented in the‘head-to-head’ or ‘tail-to-tail’ manner along the polar axis of thematerial, whereby there is in fact provided a highly stable invertedlypoled domain layer disposed at regions 9 of the wafer perpendicular tothe polar axis thereof and preventing the regions 9 from beinginfluenced by an applied electric field. If, for example, the wafer is aKTP crystal, the invertedly poled domain layer is preferably formed bymeans of Rb-indiffusion on the C⁻ polar surface 6 of the wafer. Theprocess of the formation of such a layer is described in D. Eger, M.Oron and M. Katz, J. Appl. Phys., 74, pp.4298-4302, 1993, incorporatedherein by reference. Alternatively, the bi-domain structure may befabricated by selective application of short electric pulses at theregions 9 of the wafer, or by exposing the polar surface 6 to anelectron beam or other charged particles beam, or by different kinds ofdiffusion treatment.

As seen in FIGS. 2, 3 and 4, the patterned polar surface 6 and theopposite polar surface 7 of the wafer are further coated with continuousmetallic layers which constitute switching electrodes 10 and 11, wherebythe first regions 8 contact directly with the electrode 10 and thesecond regions 9 are protected from direct contact therewith.Alternatively, the continuous electrode 10 disposed on the patternedpolar surface 6 of the wafer may be in the form of an array of separateelectrodes each directly connected with a corresponding first region 2(not shown).

As schematically shown in FIG. 5, the wafer prepared as described aboveis mounted on a temperature controlled stage 15, preferably, in a vacuumchamber 16. The electrodes 10 and 11 are connected to an electric powersource schematically designated as 20.

Subsequently, the temperature of the wafer is brought to a workingtemperature T_(w) and a pulse of a switching voltage is applied to theelectrodes 10 and 11 such as to provide the switching field of apredetermined intensity E, the pulse duration being τ_(dur) satisfyingthe condition τ_(sw)<τ_(dur)<τ_(res). Thereby, it is ensured thatdomains are completely inverted in the sections 2 of the waferassociated with the regions 8 of the polar surface 6 which are in directcontact with the electrode 10, and are not inverted in the section 3 ofthe wafer associated with the regions 9 which are protected as explainedwith reference to FIGS. 2, 3 and 4.

It should be mentioned that there exists an alternative solution tosatisfy the above condition, which is modifying the crystal so as tochange its conductivity to increase the dielectric response time τ_(res)to meet the condition τ_(res)>τ_(sw). This may be done, for example, bymeans of a method described in P. A. Morris et al., Reduction of ionicconductivity of flux grown KTiPO ₄, Journal of Crystal Growth, 109(1991), 367-375, which is incorporated herein by reference.

There will now be described how to choose, in accordance with thepresent invention, the working temperature T_(w) and the time intervalτ_(dur) during which the switching electrical field should be applied tothe wafer.

There is established a range of working temperatures T_(w) satisfyingthe condition that T_(min)<T_(w)<T_(x), where the temperature T_(min) isthe minimal temperature at which the intensity E of the switchingelectric field still satisfies the condition E_(br)>E≧E_(c), whereE_(c)=E_(c)(T) is the coercive field of the ferroelectric crystal andE_(br)=E_(br)(T) is the breakdown field of the ferroelectric crystal,and where T_(x) is the temperature below which the dielectric responsetime τ_(res)=τ_(res)(T) of the crystal is longer than the switching timeτ_(sw)=τ_(sw)(T) of the crystal at the chosen intensity E of theswitching electric field. The above mentioned parameters can bedetermined on the basis of physical measurements of the ferroelectriccrystal which may be conducted on a reference piece wafer.

In ferroelectric crystals having, at room temperature, a relatively highelectrical conductivity, the working temperature T_(w) will most oftenbe lower than room temperature and in ferroelectric crystals having, atroom temperature, a relatively low electrical conductivity, it ispreferable that the working temperature T_(w) be higher than roomtemperature.

It will now be explained, as an example, with reference to FIG. 6, howT_(w) and τ_(dur) are chosen for a flux grown KTP crystal. As seen inFIG. 6, the dielectric response time τ_(res) and the switching timeτ_(sw) depend on the temperature at which the switching electric fieldis applied to the wafer. As seen, with the intensity of the switchingelectric field being 65 kV/cm, the dielectric response time τ_(res) ofthe crystal is shorter than its switching time τ_(sw) at a temperaturehigher than about T_(x), whilst at a temperature lower than T_(x), thedielectric response time τ_(res) is longer than the switching timeτ_(sw). Thus, the switching electric field must be applied to the waferat the working temperature T_(w) which is substantially lower than thetemperature T_(x) and for the time interval τ_(dur) which is shorterthan the dielectric response time τ_(res) and at least not shorter thanthe switching time τ_(sw). However, the working temperature T_(w) mustnecessarily be higher than T_(min) defined above.

Experiments show that, to avoid back switching at the sections 2 of thewafer associated with the first, unprotected regions of the wafer and toensure that absolutely no changes occur in the polarity of the sections3 associated with the second, protected regions, it is preferable thatthe working temperature T_(w) is lower than the temperature at which theswitching time τ_(sw) equals τ_(res)/10 and the time interval τ_(dur)meets the following condition:

(1÷3)τ_(sw)≦τ_(dur)≦(0.1÷0.3)τ_(res)

The following are three examples of the use of the above describedmethod.

EXAMPLE 1

A periodically poled domain structure of a monodomain flux grown, z-cutKTP wafer was fabricated as follows. The wafer was patterned to havefirst and second spatial regions by the formation of the z⁺ polarsurface of the wafer of a photoresist patterned layer having a period ofmodulation=Λ=18 μm. Both polar surfaces of the wafer were coated withthe electrode layers of Ti having the thickness of about 1000 A. Thewafer was disposed in a vacuum chamber and its temperature was chosen tobe T_(w)−57° C. At this temperature, the conductivity of the crystal isσ=2.09·10⁻¹² Ω⁻¹ cm⁻¹ and its dielectric response time τ_(res) is 1.27s. At the applied switching electric field of 65 kV/cm, the switchingtime τ_(sw) is 2.37 ms. The time interval during which the switchingelectric field was applied to the wafer was chosen to be 5.5 ms. Theperiodically invertedly poled domain structures fabricated as above hada high quality grating of inverted domains across the entire wafer fromthe front to the back surface thereof (see FIG. 7) and presented theefficiency of the second harmonic generation close to the theoreticalone.

EXAMPLE 2

A periodically poled domain structure was fabricated similarly to thestructure in Example 1 but had a thickness −0.5 mm, a length −9 mm and aperiod 6.9 μm. FIG. 8 illustrates the intensity of a second harmonicgeneration radiation obtained by means of this structure as a functionof the wavelength of a fundamental IR radiation. The narrow width of thepeak seen at 980.5 nm and its intensity indicate that the domainpolarity inversion occurred along the entire length of the wafer. Asseen, the method according to the present invention may be potentiallyused for fabricating PPDSs having very small periods such as thoserequired for the generation of UV radiation, which can hardly beobtained with conventional techniques.

As mentioned above, the method of the present invention is useful notonly for use with the ferroelectric crystals as indicated above but alsowith the ferroelectric crystals which, at room temperature, have arelatively long dielectric time response τ_(res) but coercive fields ofwhich are very high. The use of such materials in the method of thepresent invention is associated with working temperatures higher thanroom temperature but substantially lower than the Curie temperature.

EXAMPLE 3

LiNbO₃ crystal at room temperature has very high coercive field, i.e.260 kV/cm. By raising the temperature of the crystal, for example, to130° C. or to 170° C., the coercive field may be reduced respectively to80 kV/cm or to 60 kV/cm. Although at such temperatures the electricalconductivity of the crystal increases, its dielectric response time isstill sufficiently long to meet the conditions of the present invention.There it is suggested, in accordance with the present invention, to polea LiNbO₃ wafer at temperatures between 100° C. to 200° C. Thereby,relatively thick wafers may be used for the fabrication of invertedlypoled domain structures of this kind of crystal.

Finally, invertedly poled domain structures fabricated by the method ofthe present invention may be periodic and non-periodic and may be usedfor different purposes associated with conversion of electromagneticradiation. Periodically invertedly poled domain structures fabricated bythe method of the present invention are particularly useful forproviding quasi-phase matching between at least two radiation beamspropagating within the structure, which may be used in second harmonicgenerators, sum frequency generators, difference frequency generators,optical parametric oscillators and the like. The structures may also beused for the purposes of optical switching, scanning and modulating.

What is claimed is:
 1. A method of fabricating an invertedly poleddomain structure having alternating sections of opposite electricpolarities, from a ferroelectric crystal wafer having two opposite polarsurfaces, the method comprising: (a) patterning at least one of said twopolar surfaces of the wafer to comprise a plurality of alternatingdiscrete regions, of which first regions are adapted for, and secondregions are protected from, the direct application thereto of anelectric contact; (b) applying to both polar surfaces of the waferelectrically conducting electrodes so that the first regions are indirect contact with the electrodes and the second regions are protectedfrom such a contact; (c) applying to the electrodes a switchingelectrical field of the intensity E which satisfies the conditionE_(br)>E≧E_(c), E_(c) being the coercive field of the ferroelectriccrystal and E_(br) being the breakdown field of the ferroelectriccrystal; wherein said switching electrical field is applied to the waferat a working temperature T_(w) which satisfies the conditionT_(min)<T_(w)<T_(x), where the temperature T_(min) is the minimaltemperature at which the intensity E of the switching electric fieldstill satisfies the condition E_(br)>E≧E_(c) and where T_(x) is thetemperature at which a dielectric response time τ_(res) equals aswitching time τ_(sw) of the ferroelectric crystal at the chosenintensity E of the electric field; and wherein a time interval τ_(dur)during which the switching electric field is applied to theferroelectric crystal wafer, meets the condition(1:3)τ_(sw)≦τ_(dur)≦(0.1:3)τ_(res).
 2. A method according to claim 1,wherein the operation (c) is performed with the wafer being mounted on atemperature controlled stage.
 3. A method according to claim 1, whereinthe working temperature T_(w) is lower than room temperature forferroelectric crystal wafers whose dielectric response time τ_(res) atroom temperature is shorter than their switching time τ_(sw) at thechosen intensity E of the switching electric field.
 4. A methodaccording to claim 1, wherein the working temperature T_(w) is higherthan room temperature and substantially lower than Curie temperature ofthe wafer for ferroelectric crystal wafers whose coercive field, E_(c),at temperatures above room temperature is lower than at roomtemperature.
 5. A method according to claim 1, wherein, before the step(a), the crystal of which the wafer is made is modified to change itsconductivity so as to increase the dielectric response time τ_(res) tomeet the condition τ_(res)>τ_(sw).
 6. A method according to claim 1,wherein the switching electric field applied to the wafer is in the formof a sequence of pulses the total duration of which is defined byτ_(dur).
 7. A method according to claim 1, wherein the patterning of thewafer polar surface is provided by forming thereon a patterned layer ofan isolating material.
 8. A method according to claim 1, wherein thepatterning of the wafer polar surface is provided by creating, adjacentthe polar surface, a structure of chemically modified regions.
 9. Amethod according to claim 1, wherein the patterning of the wafer polarsurface is provided by creating, adjacent the polar surface, a structureof shallow inverted domains.
 10. A method according to claim 1, whereinthe ferroelectric material is K_(1-x)Rb_(x)TiOP_(1-y)As_(y)O₄ (1≧x≧0,1≧y≧0).
 11. A method according to claim 1, wherein the ferroelectricmaterial is A_(1-x)B_(x)Ti_(1-z)Nb_(z)OP_(1-y)As_(y)O₄ (1≧x≧0, 1≧y≧0,1≧z≧0), where A and B are one of the elements: Na, K, Cs, Rb or H.
 12. Amethod according to claim 1, wherein the ferroelectric material isLiNb_(1-x)Ta_(x)O₃ (1≧x≧0).
 13. A method according to claim 1, whereinthe ferroelectric material is KNb_(1-x)Ta_(x)O₃ (1≧x≧0).
 14. A methodaccording to claim 1, wherein the invertedly poled domain structure isperiodic.
 15. A method according to claim 14, wherein the invertedlypoled domain structure is adapted for use for the conversion ofelectromagnetic radiation.
 16. A method according to claim 15, whereinthe invertedly poled domain structure is adapted for the provision ofquasi-phase matching between at least two radiation beams propagatingwithin the structure.
 17. A method according to claim 16, wherein theinvertedly poled domain structure is adapted for use in second harmonicgenerators or sum frequency generators or difference frequencygenerators or optical parametric oscillators.
 18. A method offabricating an invertedly poled domain structure having alternatingsections of opposite electric polarities, from a ferroelectric crystalwafer having two opposite polar surfaces, the method comprising: (a)patterning at least one of said two polar surfaces of the wafer tocomprise a plurality of alternating discrete regions, of which firstregions are adapted for, and second regions are protected from, thedirect application thereto of an electric contact; (b) applying to bothpolar surfaces of the wafer electrically conducting electrodes so thatthe first regions are in direct contact with the electrodes and thesecond regions are protected from such a contact; (c) applying to theelectrodes a switching electrical field of the intensity E whichsatisfies the condition E_(br)>E≧E_(c), E_(c) being the coercive fieldof the ferroelectric crystal and E_(br) being the breakdown field of theferroelectric crystal; wherein a time interval τ_(dur), during which theswitching electric field is applied to the ferroelectric crystal wafer,meets the condition (1:3)τ_(sw)≦τ_(dur)≦(0.1:0.3)τ_(res) where τ_(sw)and τ_(res) are, respectively, a switching time at the intensity E ofthe switching electric field and a dielectric response time of theferroelectric crystal wafer.
 19. A method according to claim 18, whereinthe operation (c) is performed with the wafer being mounted on atemperature controlled stage in a vacuum chamber.
 20. A method accordingto claim 18, wherein the electric voltage applied to the wafer is in theform of a sequence of pulses the total duration of which is defined byτ_(dur).
 21. A method according to claim 18, wherein the patterning ofthe wafer polar surface is provided by forming thereon a patterned layerof an isolating material.
 22. A method according to claim 18, whereinthe patterning of the wafer polar surface is provided by creating,adjacent the polar surface, a structure of chemically modified regions.23. A method according to claim 18, wherein the patterning of the waferpolar surface is provided by creating, adjacent the polar surface, astructure of shallow inverted domains.
 24. A method according to claim18, wherein the ferroelectric material isK_(1-x)Rb_(x)TiOP_(1-y)As_(y)O₄ (1≧x≧0, 1≧y≧0).
 25. A method accordingto claim 18, wherein the ferroelectric material isA_(1-x)B_(x)Ti_(1-z)Nb_(z)OP_(1-y)As_(y)O₄ (1≧x≧0, 1≧y≧0, 1≧z≧0), whereA and B are one of the elements: Na, K, Cs, Rb or H.
 26. A methodaccording to claim 18, wherein the ferroelectric material isLiNb_(1-x)Ta_(x)O₃ (1≧x≧0).
 27. A method according to claim 18, whereinthe ferroelectric material is KNb_(1-x)Ta_(x)O₃ (1≧x≧0).
 28. A methodaccording to claim 18, wherein the invertedly poled domain structure isperiodic.
 29. A method according to claim 28, wherein the invertedlypoled domain structure is adapted for use for the conversion ofelectromagnetic radiation.
 30. A method according to claim 29, whereinthe invertedly poled domain structure is adapted for the provision ofquasi-phase matching between at least two radiation beams propagatingwithin the structure.
 31. A method according to claim 30, wherein theinvertedly poled domain structure is adapted for use in second harmonicgenerators or sum frequency generators or difference frequencygenerators or optical parametric oscillators.
 32. A method ofdetermining parameters for fabricating an invertedly poled domainstructure having alternating sections of opposite electric polarities,from a ferroelectric crystal wafer having two opposite polar surfaces,the wafer being patterned at least at one of said two polar surfaces ofsaid wafer to comprise a plurality of alternating discrete regions, ofwhich first regions are adapted for, and second regions are protectedfrom, the direct application thereto of an electric contact, to enablethe applications to both of its polar surfaces of electricallyconducting electrodes so that the first regions are in direct contactwith the electrodes and the second regions are protected from such acontact; the method comprising: (a) choosing an intensity E of anelectric field for applying to said electrodes, which satisfies thecondition E_(br)>E≧E_(c), E_(c) being the coercive field of saidferroelectric crystal and E_(br) being the breakdown field of saidferroelectric crystal wafer; (b) determining the dependency of aswitching time τ_(sw) of said ferroelectric crystal wafer on temperatureat the chosen intensity E of the electric field; (c) determining atemperature T_(x) at which said switching time τ_(sw) of theferroelectric crystal wafer at the chosen intensity E of the electricfield equals a dielectric response time τ_(res) of the ferroelectriccrystal wafer; and (d) determining a working temperature T_(w) at whichsaid switching electric field applied to the wafer satisfies thecondition T_(min)<T_(w)<T_(x), where the temperature T_(min) is theminimum temperature at which the intensity E of the switching electricfield still satisfies the condition E_(br)>E>E_(c).
 33. A methodaccording to claim 32, further including a time determining interval,τ_(dur), during which the switching electric field is applied to theferroelectric crystal wafer, to meet the conditionτ_(sw)<τ_(dur)<τ_(res).
 34. A method according to claim 33, wherein thetime interval τ_(dur) meets the condition(1:3)τ_(sw)≦τ_(dur)≦(0.1:0.3)τ_(res).
 35. A method according to claim32, wherein the working temperature T_(w) is lower than room temperaturefor ferroelectric crystal whose dielectric response time τ_(res) at roomtemperature is shorter than their switching time τ_(sw) at the chosenintensity E of the switching electric field.
 36. A method according toclaim 32, wherein the working temperature T_(w) is higher than roomtemperature and substantially lower than Curie temperature of the waferfor ferroelectric crystal wafers whose coercive field, E_(c), attemperatures above room temperature is lower than at room temperature.37. A method according to claim 32, wherein the ferroelectric crystalmaterial is selected from LiNb_(1-x)Ta_(x)O₃, KNb_(1-x)Ta_(x)O₃,K_(1-x)Rb_(x)TiOP_(1-y)As_(y)O₄, orA_(1-x)B_(x)Ti_(1-z)Nb_(z)OP_(1-y)As_(y)O₄, where A and B are one of theelements: Na, K, Cs, Rb or H, and where x, y, and z are eachindependently a fraction between 0 and 1, inclusive.
 38. A method offabricating an invertedly poled domain structure having alternatingsections of opposite electric polarities, from a ferroelectric crystalwafer having two opposite polar surfaces, the method comprising: (a)providing said wafer made of a ferroelectric material whose dielectricresponse time τ_(res) is shorter than the switching time τ_(sw) at roomtemperature, and is equal to said switching time at a temperature T_(x)lower than room temperature, at an intensity E of a switching electricalfield which satisfies the condition E_(br)>E>E_(c), E_(c) being thecoercive field of the ferroelectric crystal and E_(br) being thebreakdown field of the ferroelectric crystal; (b) patterning at leastone of said two polar surfaces of said wafer to comprise a plurality ofalternating discrete regions, of which first regions are adapted for,and second regions are protected from, the direct application thereto ofan electric contact; (c) applying to both polar surfaces of the waferelectrically conducting electrodes so that the first regions are indirect contact with the electrodes and the second regions are protectedfrom such a contact; (d) applying to the electrodes a switchingelectrical field of said intensity E wherein said switching electricalfield is applied to the wafer at a working temperature T_(w) whichsatisfies the condition T_(min)<T_(w)<T_(x), where the temperatureT_(min) is the minimal temperature at which the intensity E of theswitching electric field still satisfies the condition E_(br)>E≧E_(c).39. A method according to claim 38, wherein, before the step (b), thecrystal of which the wafer is made, is modified to change itsconductivity so as to increase the dielectric response time τ_(res) tomeet the condition τ_(res)>τ_(sw) in step (a).
 40. A method according toclaim 38, wherein the ferroelectric material isK_(1-x)Rb_(x)TiOP_(1-y)As_(y)O₄ (1≧x≧0, 1≧y≧0).
 41. A method accordingto claim 38, wherein the ferroelectric material isA_(1-x)B_(x)Ti_(1-z)Nb_(z)OP_(1-y)As_(y)O₄ (1≧x≧0, 1≧y≧0, 1≧z≧0), whereA and B are one of the elements: Na, K, Cs, Rb or H.
 42. A methodaccording to claim 38, wherein the ferroelectric material isKNb_(1-x)Ta_(x)O₃ (1≧x≧0).
 43. A method according to claim 38, furtherincluding a time determining interval, τ_(dur), during which theswitching electric field is applied to the ferroelectric crystal wafer,to meet the condition τ_(sw)<τ_(dur)<τ_(res).
 44. A method according toclaim 43, wherein the time interval τ_(dur) meets the condition(1:3)τ_(sw)≦τ_(dur)≦(0.1:0.3)τ_(res).
 45. A method of fabricating aninvertedly poled domain structure having alternating sections ofopposite electric polarities, from a ferroelectric crystal wafer havingtwo opposite polar surfaces, the method comprising: (a) providing saidwafer made of a ferroelectric crystal having at room temperature adielectric response time τ_(res) shorter than its switching time τ_(sw);(b) modifying said crystal to change its conductivity so as to increasethe dielectric response time τ_(res) to meet the conditionτ_(res)>τ_(sw); (c) patterning at least one of said two polar surfacesof the wafer to comprise a plurality of alternating discrete regions, ofwhich first regions are adapted for, and second regions are protectedfrom, the direct application thereto of an electric contact; (d)applying to both polar surfaces of the wafer electrically conductingelectrodes so that the first regions are in direct contact with theelectrodes and the second regions are protected from such a contact; (e)applying to the electrodes a switching electrical field of the intensityE which satisfies the condition E_(br)>E≧E_(c), E_(c) being the coercivefield of the ferroelectric crystal and E_(br) being the breakdown fieldof the ferroelectric crystal; wherein said switching electrical field isapplied to the wafer at a working temperature T_(w) which satisfies thecondition T_(min)<T_(w)<T_(x), where the temperature T_(min) is theminimal temperature at which the intensity E of the switching electricfield still satisfies the condition E_(br)>E≧E_(c) and where T_(x) isthe temperature at which said dielectric response time τ_(res) equalssaid switching time τ_(sw) of the ferroelectric crystal at the chosenintensity E of the electric field.
 46. A method according to claim 45,further including a time determining interval, τ_(dur), during which theswitching electric field is applied to the ferroelectric crystal wafer,to meet the condition τ_(sw)<τ_(dur)<τ_(res).
 47. A method according toclaim 46, wherein the time interval τ_(dur) meets the condition:(1:3)τ_(sw)≦τ_(dur)≦(0.1:0.3)τ_(res).